The invention concerns a detachable assembly for spot feeding alumina to an electrolytic tank for the production of aluminum by the Hall-Heroult process.
In recent years the operation of tanks producing aluminum has been gradually automated, thereby improving the energy balance and the smoothness of operation, restricting human intervention, and improving the recovery of fluorine-containing effluent.
One of the essential factors in ensuring the smooth running of a tank producing aluminum by electrolysis of alumina dissolved in molten cryolite is the timing of the introduction of alumina into the bath.
Until recently the alumina was fed in by depositing it on the solidified electrolytic crust covering the tank and by periodically breaking the crust with picks. This caused the alumina to drop into the bath in large and uncontrolled quantities.
Processes of this type have been described, for example, in U.S. Pat. Nos. 3,216,918; 3,372,106; and 3,679,555. These patents correspond respectively to French Pat. Nos. 1,245,598; 1,526,766; and 2,036,896.
The tendency now is to feed in the alumina in a controlled and regular manner to keep the alumina concentration of the electrolyte permanently within a predetermined range with a view to obtaining the best possible yield.
For this purpose the alumina is fed in successive doses of controlled weight into one or more holes which are kept permanently open in the electrolyte crust (a system known as "spot feeding"). Systems of this type have been described particularly in French Pat. Nos. 2,099,434 (ALCOA), 2,264,098 (NIPPON LIGHT METALS), 2,465,016 (ALUSUISSE), 2,483,964, and 2,483,965 (ALUMINIUM DE GRECE), and 2,487,386 (ALUMINIUM PECHINEY), and also in U.S. Pat. Nos. 3,400,062 (ALCOA), and 3,689,229 (ALCOA). Such systems generally consist of storage means and a means for dispensing alumina into the tanks at one or more points, the alumina dispensing means frequently being combined with the means for forming a hole in the crust of solidified electrolyte.
The storage means may be either of the centralized type with the alumina being distributed into each tank by an air-slide or pneumatic or fluidizing conveyor, or a localized type with one or more hoppers per tank which are periodically recharged.
The disadvantage of these systems, particularly when the hopper is fixed, resides in the difficulty of gaining access to the means for breaking the crust and dosing and dispensing alumina. Because of the abrasive nature of the product, these means require relatively frequent maintenance which has to be carried out on the tank under very difficult conditions, arising from the heat, the liberation of fluorine-containing gases, electric potential on the superstructure, and strong magnetic fields which cause the tools to stick and make handling difficult.
As a means of resolving this problem it has been proposed that the hopper, or the unit comprising the hopper, the crust breaker and the dosing means, should be made detachable or movable on a rail integral with the superstructure.
However, the detachable hopper system has many disadvantages. It is expensive to set up since it necessitates making large items interchangeable, thus requiring very small manufacturing tolerances. Further, outside the tanks it is necessary to provide for storage of exchange hoppers and hoppers being repaired, which require appropriate supports and take up space in the building.
In addition, the steel of the hopper does not contribute toward the strength of the superstructure of the tank or toward forming the hoods for sucking in the gases. The capacity of detachable hoppers is very substantially reduced (up to 30%) by the play or clearances necessary for the exchanging processes. The hoppers therefore have to be filled frequently and operation is less reliable.
When a worn out hopper is being exchanged or a hopper dismantled to gain access to the crust breaking and dosing means, the alumina has to be emptied out, then the replacement hopper filled with alumina to bring it into operation. These transfers of alumina make dust fly, which is prejudicial to the proper operation of the tank mechanisms and to working conditions. Such problems lengthen the time taken by the exchange and thus the time during which the tank is not being fed with alumina.
Furthermore, the connection between the detachable hopper and the superstructure of the tank must be as impervious as possible to the tank gases, in order to increase the amount recovered. Such imperviousness is difficult to maintain with large components of the type involved.
Finally, an inevitable consequence of the use of a detachable hopper is the presence of alumina in the maintenance workshop, since it is not possible to empty the hoppers completely. Special precautions therefore have to be taken throughout in order to prevent the tools, machines and the parts being maintained from being contaminated with the highly abrasive alumina.